1. Field of the Invention
The present invention relates to a fibrous material for the manufacture of fillings for elastic calender rolls, for example, for supercalenders for paper glazing and also relates to elastic calender rolls provided with a filling consisting of compressed fibrous material in combination with carbon fibers.
2. Description of the Prior Art
So-called supercalenders are employed for glazing, i.e., for calendering high-grade printing papers as well as other special papers such as pergamyne (vegetable parchment). The supercalenders consist of a set of successive rolls which form pressure gaps with one another and essentially consist of an alternating series of hard steel rolls and of rolls having a thick elastic jacket which is deformed under the pressure of the pressing gap. The paper is successively conducted back and forth through the individual pressing gaps and is calendered as a result of the speed difference present and also as a result of the temperature produced by the fulling action of the elastic rolls.
A special fibrous material which is passed onto roll cores under high pressures of about 500 to 600 bar and is subsequently cylindrically turned to size and burnished has prevailed as the predominating material for the jacket or filling of the elastic rolls of supercalender assemblies. Cellulose fibers, particularly cotton linters, are usually employed as the fibrous material. These cellulose fibers can, however, have other fibrous materials added to them. Thus, for example, the European standard filling for elastic calender rolls consists of 80% cotton and 20% wool fibers. Roll fillings containing up to 50% asbestos fibers can also be utilized for special purposes.
The fibrous material employed for filling elastic calender rolls and consisting essentially of cotton fibers with possibly some wool fibers in the majority of cases is employed in the form of a non-woven web which is manufactured according to traditional paper manufacturing methods on endless wire machines. Octagonal or round disks having a center opening for the roll core cut from the fibrous web thus produced, are then stacked on the roll core and compressed in the axial direction with pressures of up to 600 bar. The rolls processed in this manner can then be turned to size and burnished.
It is not absolutely necessary to make the fibrous material for the roll filling available in the form of a paper-like fibrous web. Manufacturing methods are also known wherein the fiber material such as carded cotton fiber is pressed into the roll core in some other manner. At present, however, calender roll compositions in the form of paper-like webs are nearly exclusively employed for recoating elastic calender rolls.
Cellulose fibers, particularly cotton linters, utilized for the filling of elastic calender rolls offer improved technical properties for calendering the papers to be processed which accounts for their widespread employment. However, they cause a number of potential and generally cost-increasing difficulties for operating the calenders. Considerably high temperatures are produced in performing the fulling function at the circumferential region of the rolls, with the considerable line pressures of up to 300 daN/cm which are frequently employed. Considering the relatively poor thermal conductivity of the cellulose material of the cotton fibers, a heat build-up due to non-dissipated thermal energy arises in the roll jackets, the build-up leading to the highest temperatures in a region at about 10 mm below the roll surface. In particular, temperature peaks occur in the area causing superficial damage to the rolls, such damage easily giving rise to tearing of the glazed paper web or permitting the passage of foreign bodies through the roll gaps. The elevated temperatures occur particularly at such locations that the fibrous material of the roll filling actually burns below the surface. As a result, the roll filling loses its specific properties in these regions and generally becomes unusable for further employment. When this occurs, considerable costs are incurred for recoating.
A number of structural measures have been tried in calenders in order to prevent temperature peaks that lead to roll scorching from occurring. One such measure is the use of internal roll cooling. Considering the poor thermal conductivity of the cellulose material, however, such measures have only a limited effect. The difficulties involved as well as measures that have been tried to eliminate these difficulties are described, for example, by E. Munch and W. Schmitz in the "Wochenblatt fur Papierfabrikation" 1980, Number 11/12. In this publication, the expert authors confirm that the technological possibilities of a supercalender could not hitherto be exploited because of the danger of scorching the elastic rolls which has not yet been controlled. Since calenders for special papers such as pergamyne require a very high glaze, calender roll fillings containing up to 50% asbestos fibers have been employed because these fibers resist the high temperatures to a larger extent. In terms of their other physical properties, however, such roll fillings are not as beneficial. Further attempts have been undertaken to find heat resistant fiber materials for calender roll fillings which equal the cotton coatings with respect to technical properties. Up to now, however, these efforts have been unsuccessful.